A capacitively coupled (parallel plate) plasma processing apparatus, an inductively coupled plasma processing apparatus, a microwave plasma processing apparatus or the like has been put to practical use in an apparatus for performing a micro process such as an etching process or a film forming process on a target object to be processed by using a plasma. Especially, the parallel plate type plasma processing apparatus generates an electric field energy by applying a high frequency power to at least one of an upper electrode and a lower electrode facing each other. The generated electric field energy excites a gas to generate a plasma, thereby performing the micro process on the target object.
Along with recent demands for the device scaling-down, it becomes necessary to supply a relatively high frequency power of about 100 MHz to generate a high-density plasma. As the frequency of the supplied power becomes higher, a high frequency current flows from an end portion to a central portion on the plasma-side surface of the electrode due to the skin effect.
Thus, as shown in FIG. 3C, the electric field strength E becomes higher at the central portion of the electrode 105 than at the end portion of the electrode 105. Hence, the more electric field energy can be consumed for plasma generation at the central portion of the electrode than at the end portion of the electrode, which leads to more ionization or dissociation of a gas at the central portion of the electrode than at the end portion.
Accordingly, the plasma electron density Ne at the central portion of the electrode becomes higher than that at the end portion thereof. The resistivity of the plasma is decreased at the central portion of the electrode with the higher electron density Ne, so that a current with a high frequency (electromagnetic wave) is concentrated on a central portion of the facing electrode, which further results in non-uniformity of the plasma density.
In order to improve the uniformity in the plasma density, there is suggested a method of burying a dielectric material, e.g., ceramics, in the central portion of the plasma-side surface of the electrode (see, e.g., Japanese Patent Application Publication No. 2004-363522). With such method, further, the dielectric body 205 buried in the electrode 105 has a tapered portion with a gradually reduced thickness from its central portion toward its periphery, as can be seen from FIG. 3B.
In that case, a capacitance component becomes larger at an end portion of the dielectric body 205 than at the central portion thereof, so that the electric field strength E is not excessively lowered at the end portion of the dielectric body 205 as compared with a case where a flat dielectric body is buried. As a result, it is possible to improve the uniformity in the electric field strength distribution.
Meanwhile, in the electrode 105 shown in FIG. 3B, an upper base 105a is formed of a metal and the dielectric body 205 is formed of ceramics. Therefore, if heating and cooling during processing are repetitively performed in a state where the tapered dielectric body 205 is buried in the upper base 105a, a difference in thermal expansion of the upper base 105a and the dielectric body 205 causes stress concentration at a bonding portion therebetween. Accordingly, there occur problems such as development of cracks in the electrode 105 and contamination of the chamber.
In consideration of this, a proper gap may be provided between the dielectric body 205 and the upper base 105a. However, if the dielectric body 205 has a tapered shape, the dimensional accuracy is deteriorated at the tapered portion due to a lack of machining accuracy. This results in stress concentration at the tapered portion of the dielectric body 205 due to a difference in thermal expansion.
The stress concentration is also caused by a difference in thermal conductivity due to a tapering thickness of the dielectric body 205 or a discrepancy in dimensional tolerance of the gap. It may also cause cracks of the upper electrode 105 and contamination of the chamber.